The present invention relates in general to electric fuel pumps, and, more specifically, to reducing films that build-up on carbon-based commutators and brushes during exposure to fuel.
One conventional type of automotive fuel pump uses an electric motor immersed in the fuel inside a pump housing to drive an impeller or a roller mechanism to pump fuel from a fuel tank to an engine in a vehicle. Fuel flowing through the motor advantageously cools the motor during operation. By not sealing the motor components from the fuel, a more inexpensive and compact pump design is achieved.
The pump motor typically comprises a DC motor having a commutator and brushes for coupling current to armature coils. Efficient coupling of current between the brushes and commutator depends on maintaining robust contact between them. The contact of fuel with the brush-commutators, however, results in the buildup of various high resistance materials on the brush-commutator interface referred to as filming. The increased resistance of the connection between the brushes and commutator reduces current flow to the armature thereby reducing the flow rate through the pump. The reduced flow rate impacts engine performance and may require a pump to be replaced.
The rate at which filming occurs may vary depending upon the type of fuel present. Modern vehicles are typically exposed to various grades and types of fuel. Ethanol/gasoline blends such as E10 fuel may have a particularly high rate of filming. As these fuels are increasingly used, the problem of filming is becoming more urgent.
The rate of filming also depends upon the material used for constructing the brush and the commutator. One traditional commutator material has been copper. Although copper is less susceptible to film formation than some other materials, the surface of the copper wears away at an undesirably high rate. While the wearing away of the copper surface is probably responsible for the lower amount of filming, the premature wearing away of the commutator provides a shortened service life of the pump motor. Thicker commutator pads could provide greater lifetime, but would undesirably increase the length and mass of the armature thereby decreasing efficiency. Fuel pump brushes typically have been and continue to be made of carbon and carbon-based materials.
More recently, carbon-based materials have been used for commutators because of their increased wear resistance. These carbon-based materials may include sintered carbon or carbon mixed with resins or other materials. A disadvantage of the carbon-based materials is an increased susceptibility to buildup of a filming layer. One solution has been to apply various coatings to the commutator and/or brush comprising a material more resistant to buildup of the filming layer. However, these measures have resulted in significantly increased costs of materials and cost of manufacture. Therefore, it would be desirable to reduce filming without requiring special materials or manufacturing processes.